Corticosteroid-immunosuppressed animals and AIDS patients share a remarkable susceptibility to pneumonia caused by Pneumocystis carinii (PC). Such susceptibility may be due to a decrease in production or secretion of microbicidal mediators in the lung. The microbicidal substances of lung phagocytes (superoxide anion, H2O2, lysozyme, cationic peptides) will be quantitated in normal and corticosteroid-immunosuppressed rabbits. The microbicidal activity of such substances against PC will be measured using the cessation of radiolabeled precursor uptake and lysosomotrophic dye uptake by PC as an index of their loss of viability. The modulation of the levels of microbicidal substances by lymphokines or other materials present in lung secretions will be explored.